Re: [PATCH v3 4/6] xen/cpupool: Create different cpupools at boot time

[Julien Grall <julien@xxxxxxx>]

Hi,

On 21/03/2022 15:58, Luca Fancellu wrote:
> > On 18 Mar 2022, at 16:12, Julien Grall <julien@xxxxxxx> wrote:
> >
> > Hi Luca,
> >
> > I only skimmed through the series. I have one question below:
> >
> > On 18/03/2022 15:25, Luca Fancellu wrote:
> > > +void __init btcpupools_allocate_pools(void)
> > > +{
> > > +    unsigned int i;
> > > +    bool add_extra_cpupool = false;
> > > +
> > > +    /*
> > > +     * If there are no cpupools, the value of next_pool_id is zero, so the code
> > > +     * below will assign every cpu to cpupool0 as the default behavior.
> > > +     * When there are cpupools, the code below is assigning all the not
> > > +     * assigned cpu to a new pool (next_pool_id value is the last id + 1).
> > > +     * In the same loop we check if there is any assigned cpu that is not
> > > +     * online.
> > > +     */
> > > +    for ( i = 0; i < nr_cpu_ids; i++ )
> > > +        if ( cpumask_test_cpu(i, &cpu_online_map) )
> > > +        {
> > > +            /* Unassigned cpu gets next_pool_id pool id value */
> > > +            if ( pool_cpu_map[i] < 0 )
> > > +            {
> > > +                pool_cpu_map[i] = next_pool_id;
> > > +                add_extra_cpupool = true;
> > > +            }
> > > +            printk(XENLOG_INFO "Logical CPU %u in Pool-%u.\n", i,
> > > +                   pool_cpu_map[i]);
> > > +        }
> > > +        else
> > > +        {
> > > +            if ( pool_cpu_map[i] >= 0 )
> > > +                panic("Pool-%d contains cpu%u that is not online!\n",
> > > +                      pool_cpu_map[i], i);
> > > +        }
> > > +
> > > +    if ( add_extra_cpupool )
> > > +        next_pool_id++;
> > > +
> > > +    /* Create cpupools with selected schedulers */
> > > +    for ( i = 0; i < next_pool_id; i++ )
> > > +        cpupool_create_pool(i, pool_sched_map[i]);
> > > +
> > > +#ifdef CONFIG_X86
> > > +    /* Cpu0 must be in cpupool0 for x86 */
> > > +    if ( pool_cpu_map[0] != 0 )
> > > +        panic("Cpu0 must be in Pool-0\n");
> > > +#endif
> > Can you document why this is necessary on x86 but not on other architectures?
> Hi Julien,
>
> I received the warning by Juergen here: 
> https://patchwork.kernel.org/comment/24740762/ that at least on x86 there could 
> be
> some problems if cpu0 is not in cpupool0, I tested it on arm and it was working fine 
> and I didn’t find any restriction.
What exactly did you test on Arm?

> So I don’t know why on x86 we must have cpu0 in cpupool0, maybe x86 maintainer 
> have more knowledge about that and
> I can put a comment here.
On Arm, we are not yet supporting all the CPU features that x86 supports 
(e.g. CPU hotplug, suspend/resume...). So I a am bit concerned that the 
restriction is just not there yet (or possibly hidden).
Therefore, before lifting the restriction on Arm (and other arch), I 
would like us to understand why it is necessary on x86.
We may not have the answer quickly, so is it going to be a problem to 
keep the restriction on Arm?
Cheers,

--
Julien Grall